The present invention relates to a method for producing an ethylene-vinyl alcohol copolymer (hereinafter abbreviated as xe2x80x9cEVOH xe2x80x9d) resin composition.
EVOH is a useful polymer excellent in oxygen shielding property, oil resistance, antistatic property, mechanical strength and the like, and is used widely as a packaging material in the form of a film, a sheet, a container or the like. Usually, such packages are produced by melt molding of EVOH. Thus, in EVOH, importance is given to a long-run property at the time of melt molding, which is an ability to withstand a long molding run with few fish eyes or stripes, as well as appearance characteristics of molded products (little coloration, inhibition of generation of fish eye).
To provide EVOH with such characteristics, it is known to add acids or metal salts to EVOH (JP 64(1989)-66262A), and to add a boron compound to EVOH (JP 49(1974)-20615B). Furthermore, as methods for treatment with these additives, JP 11(1999)-292929A proposes a method in which EVOH is treated by causing an aqueous solution of a treating agent to spout from a nozzle while allowing EVOH to flow. Furthermore, JP 11(1999)-152307A proposes a method in which EVOH is brought into contact with an aqueous solution of an additive in a treatment column (tower type device) while the EVOH and the aqueous solution flow in countercurrent to each other. In this method, because the EVOH is treated continuously using a treatment column, the treatment can be carried out more efficiently than a treatment of batch type. Furthermore, unevenness in the treatment can be reduced, compared to a treatment of batch type, where a solution is prepared each time.
When EVOH is treated with a boron compound such as boric acid among the above-mentioned treating agents, particularly the effect of improvement in the thermal stability of EVOH can be obtained. However, when the method of continuous treatment disclosed in JP 11(1999)-152307A is applied to a treatment of EVOH using a boron compound, many fish eyes are generated at the time of melt molding, and also the period of continuous operation is limited.
It is therefore desirable to provide a method for producing an EVOH resin composition that addresses the above-mentioned problems.
The inventors have now surprisingly found that the above-mentioned problems are caused by uneven treatment of EVOH resulting from high adsorptivity of boron compounds. Thus, viewed from one aspect, the invention provides a method for producing an ethylene-vinyl alcohol copolymer resin composition, which method comprises contacting particles of an ethylene-vinyl alcohol copolymer with a solution containing a boron compound whereby to obtain a substantially homogeneous distribution of said boron compound within said particles. As used herein, the term xe2x80x9chomogeneousxe2x80x9d is intended to encompass any substantially uniform distribution of boron compound within the polymer particle.
Preferably, the method for producing an EVOH resin composition of the present invention includes bringing EVOH into contact with a solution containing a boron compound, wherein EVOH and the solution are continuously fed into a treatment apparatus and brought into contact with each other in the treatment apparatus. EVOH is then continuously removed from the apparatus in contact with a solution containing the boron compound in a concentration of at least 0.7 times but not more than 1.3 times a solution equilibrium concentration, thereby obtaining an EVOH resin composition having a concentration of boron compound substantially at a resin equilibrium concentration corresponding to the solution equilibrium concentration.
It is herein defined that the solution equilibrium concentration (CeqSL) and the resin equilibrium concentration (CeqEV) are concentrations of the boron compound in the solution and in EVOH, respectively, achieved when EVOH is soaked in the solution and the boron compound in EVOH equilibrates with the boron compound in the solution. With respect to a resin of a given composition, there is a relationship between CeqSL and CeqEV such that when one is determined, the other is fixed. As used herein, xe2x80x9can EVOH resin compositionxe2x80x9d refers to EVOH treated with a solution containing a boron compound. In another aspect of the invention, EVOH and the solution are continuously fed into a treatment apparatus and brought into contact with each other in the treatment apparatus. EVOH is then continuously removed from the apparatus in contact with a solution containing the boron compound in a concentration of at least 0.7 times but not more than 1.3 times a solution equilibrium concentration, thereby obtaining an EVOH resin composition having a concentration of boron compound substantially at a resin equilibrium concentration corresponding to the solution equilibrium concentration.